1. Field of the Invention
The present invention generally relates to radio frequency antennas and, more particularly, to planar antennas for very low frequency, short range (near field) applications.
2. Description of the Prior Art
Antennas which function at low radio frequencies (RF) are now required for wireless devices, such as wireless peripherals. One example of a proposed application for a low frequency antenna is a wireless keyboard for a computer workstation. Such a keyboard eliminates the cable interconnecting a conventional keyboard with the system unit, providing the user with more freedom of placement of the keyboard. Infrared (IR) transmitters have been used for wireless keyboards, but while eliminating the cable, these require that there be a clear optical path between the keyboard and the system unit. Other peripherals which are candidates for wireless interconnections are printers, modems and local area networks (LANs), all of which would benefit from an elimination of the cabling usually associated with such devices and systems.
These wireless applications are all characterized by low power output and, therefore, low range (typically near field) communications. At frequencies such as 49 Mhz, the wavelength is 6.12 meters in free space. A monopole can be designed with many windings in order to create a quarter wavelength antenna. However, this may not be a desirable form factor from an aesthetic sense. Generally, it is desirable to conceal the antenna in the device packaging so that it is not visible. Furthermore, a monopole antenna can break off easily, making it undesirable from a reliability point of view. If an electrically small antenna were used, the input impedance would be capacitive, and the real part of the impedance would be very small. A matching network would have to be designed to match the previous stage, and much of the power would be lost in the matching network.